To get on the ballot paper, nominees need the support of three Labour affiliates, including at least two unions, which equates to at least fiver per cent of affiliate members.

Otherwise, an alternative route involves receiving nominations from at least five per cent of constituency Labour parties.

The union's general secretary Paddy Lillis said: "The Labour Party must be led by someone who can persuade voters that they have what it takes to be a prime minister and we are a government in waiting."

Angela Rayner has also received an endorsement from Usdaw.

It is now too late for people to join the party in order to vote in the leadership contest.

The winner will be announced on 4 April.

Labour leadership contenders at a hustings in Liverpool.
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Shadow business secretary Rebecca Long-Bailey, shadow foreign secretary Emily Thornberry and backbenchers Lisa Nandy and Jess Phillips are all still battling to join Sir Keir on the ballot.

Ms Phillips, the MP for Birmingham Yardley, has said Labour needs to stop appeasing "fear-mongering opportunists" who encourage opposition to immigration.

"For too long, we have allowed this debate to be dominated by fear-mongering opportunists: Nigel Farage standing in front of a poster featuring refugees and the words 'Breaking Point'; Michael Gove whipping up fear of mass migration from Turkey," she wrote in The Independent.

"In an attempt to sound tough, Labour has sometimes been too slow to stand up against this kind of rhetoric and, in the worst cases, even been seen to appease it.

Ms Phillips said she would offer "a different approach".
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"The most pernicious myth we have failed to bust is that working-class communities are hostile to immigration, which means that middle-class politicians must watch their language or, worse, pander to this perceived prejudice.

"This then leads to political arm-wrestling between the left and the right over who can sound the toughest on immigration. Why, when we're asked whether immigration is too high, do Labour politicians so often fumble their response? Let's say what we think: no, it isn't."

She backed a series of reforms to a system she described as "one of the cruellest and maddest parts of government."

Ms Phillips also admitted it would be "bold" for Labour members to choose her as party leader.

"A different approach. A bold roll of the dice. And I think that is what I am. And that that is the only way we can beat Boris Johnson," she said in a webchat on the website Mumsnet.